PROJECT SUMMARY Quantitative systems pharmacology (QSP) is a rapidly expanding area that integrates available in vitro, preclinical, and clinical data representing existing knowledge to achieve a reverse translation. Reverse translation uses real-time human clinical data to directly inform new discoveries, of existing therapies and attributes of disease progression. Here, we seek to be the first to build a QSP model to, a priori, predict interpatient pharmacokinetics and pharmacodynamics using population priors (population pharmacokinetic or popPK modeling) and physiologic predictions (using physiologically based pharmacokinetic or PBPK modeling) combined with pharmacodynamic models based on in vitro and preclinical data. We will apply this novel hybrid popPK-PBPK-PD QSP model to allogeneic hematopoietic cell transplant (HCT) because its success requires a delicate balance as the grafting of cells from one individual (donor) to another (host, the HCT recipient). Guided by our preliminary data, our working hypothesis is that QSP modeling can minimize interindividual variability of these immunosuppressants, while also optimizing the novel graft versus host disease (GVHD) regimen of post- transplant cyclophosphamide (PTCy). Aim 1 seeks to identify the optimal PTCy dose using popPK and PBPK models. Our preclinical data shows that PTCy has a narrow dose window, with intermediate doses having the lowest GVHD rates. To achieve the optimal PTCy dose in each patient, we seek to develop a validate a popPK- PBPK model building upon our unique expertise in quantitating 4-hydroxycyclophosphamide (4HCY), the primary precursor to the cytotoxic metabolite of CY and personalizing CY using popPK-guided dosing. This hybrid popPK-PBPK CY model will be developed using our retrospective and prospective (n=150) cohort. The prospective cohort will be enrolled at National Cancer Institute (NCI) and City of Hope (COH). The NCI cohort will determine if the PTCy dose and schedule can be reduced (by 75%) without compromising GVHD rates; the COH cohort will use the traditional PTCy dosing. In Aim 2, we will characterize the pharmacokinetics and pharmacodynamics of mycophenolic acid (MPA), the active metabolite of MMF, with its target enzyme inosine monophosphate dehydrogenase (IMPDH). Like CY, MPA has substantive pharmacokinetic variability but different metabolic and transport pathways so separate pharmacokinetic models are needed. We seek to create a popPK-PBPK-PD model to identify the optimal plasma exposure of MPA and IMPDH activity. In Aim 3, we will create a quantitative systems pharmacology (QSP) model of T-cell response and acute GVHD. Our preclinical data show that acute GVHD prevention with PTCy is associated with reduction of CD4+CD25-Foxp3- conventional T-cell (Tcon) proliferation at day +7 followed by the preferential expansion of CD4+CD25+Foxp3+ regulatory T cells (Tregs) at day +21. Building upon fully-integrated immune response model (FIRM), we seek integrate in vitro, preclinical, and clinical data to build a QSP model.